力扣216（组合总和）

216、组合总和II

基本思想：

回溯法

具体实现：

剪枝优化：

1.元素总和大于目标值，在递归终止的地方剪枝

2.和77题思路一样，for循环的范围剪枝

代码：

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    LinkedList<Integer> path = new LinkedList<>();
    public List<List<Integer>> combinationSum3(int k, int n) {
        backTracking(n, k, 1, 0);
        return result;
    }
    private void backTracking(int targrtSum, int k, int startIndex, int sum){
        if(sum > targrtSum){
            return;//剪枝
        }
        if (path.size() == k){
            if (sum == targrtSum) {
                result.add(new ArrayList<>(path));
            }
            return;
        }
        for (int i = startIndex; i <= 9 - (k - path.size()) + 1; i++){
            path.add(i);
            sum += i;
            backTracking(targrtSum,k,i+1,sum);
            path.removeLast();
            sum -= i;
        }
    }
}

39、组合总和

具体实现：

代码：

class Solution:
    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
        result = []
        track = []
        Sum = 0
        self.backtrack(candidates,target,0,result,track,Sum)
        return result
    def backtrack(self,candidates,target,start,result,track,Sum):
        if  Sum == target:
            result.append(track[:])
            return
        if  Sum > target:
            return
        for i in range(start,len(candidates)):
            track.append(candidates[i])
            Sum += candidates[i]
            self.backtrack(candidates,target,i,result,track,Sum)
            Sum -= candidates[i]
            track.pop()

剪枝：

先对candidates进行从小到大的排序

如果下一层的sum，也就是本层的sum+candidates[i] >traget,就跳出循环

代码：

class Solution:
    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
        result = []
        track = []
        Sum = 0
        candidates.sort()
        self.backtrack(candidates,target,0,result,track,Sum)
        return result
    def backtrack(self,candidates,target,start,result,track,Sum):
        if  Sum == target:
            result.append(track[:])
            return
        i = start
        while   i < len(candidates):
            if Sum + candidates[i] <= target:
                track.append(candidates[i])
                Sum += candidates[i]
                self.backtrack(candidates,target,i,result,track,Sum)
                Sum -= candidates[i]
                track.pop()
                i += 1
            else:
                break